1. Field of the Invention
The present invention relates generally to novolak resins, and, in particular, to novolak resins having lowered hydroxyl content and which are prepared using a mixture of aldehydes.
The present invention also relates generally to radiation-sensitive positive photoresist compositions and particularly to such compositions containing a novolak resin having lowered hydroxyl content and which are prepared from a mixture of aldehydes.
2. Description of the Prior Art
Positive photoresist formulations such as are described in, for example, U.S. Pat. Nos. 3,666,473; 4,115,128; 4,173,470; 4,377,631; 4,536,465; and 4,529,682, include alkali-soluble phenol-formaldehyde or cresol-formaldehyde resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied in a thin film or coating to a substrate suitable for the particular application desired.
The novolak resin component of these photoresist formulations is soluble in alkaline aqueous solution, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a radiation-induced structural transformation which decreases its efficiency as a dissolution rate inhibitor for the novolak and, subsequently, the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern of photoresist on the substrate.
Positive photoresists, especially those used to prepare microelectronic silicon wafers and chips, are often subjected to temperatures during the manufacture of the finished article which are high enough to have a deleterious effect on the photoresist. Thus, positive photoresists having improved thermal stability have long been sought. However, it is also very important that the photoresist be capable of providing high optical resolution so that precise patterns can be applied to the substrate. While positive photoresists having high resolution and contrast characteristics, such as DYNALITH.RTM. EPR-5000 resist sold by the Dynachem Division of Morton International, are known, and positive photoresists having good thermal stability, such as DYNALITH.RTM. OFPR-800 resist also sold by the Dynachem Division of Morton International, are also known, the art has yet to develop a positive photoresist which combines these thermal stability and high resolution and high contrast characteristics. In fact, formulation of a positive photoresist to enhance one of these characteristics normally adversely affects the other characteristic, i.e. photoresists with good thermal stability do not provide high resolution and high contrast and vice versa.
It was discovered and disclosed in earlier U.S. patent application Ser. No. 376,971, filed Jul. 6, 1989, issued as U.S. Pat. No. 4,943,511 on Jul. 24,1990 that positive photoresists having both a high degree of thermal stability and good resolution could be made when the novolak resin employed is prepared from a mixture of aldehydes comprising formaldehyde (or a formaldehyde precursor) and a monohydroxy aromatic aldehyde. However, while such positive photoresists had better thermal stability than the aforementioned DYNALITH.RTM. EPR-5000 resist they generally were faster and lost more film in standard strength developers. Thus, it became desirable to develop positive photoresists that could maintain the improved thermal stability of the mixed aldehyde photoresists disclosed in said U.S. Pat. No. 4,943,511 while producing photoresists having still better photospeed and film loss characteristics. It was also desirable that positive photoresists be available that are "process tunable", i.e. positive photoresist compositions in which the photospeed characteristics of the photoresist composition can be made readily and easily adjustable and whereby the photoresists can be made to perform optionally in a wide variety of developers.